Surface plasmon resonance is an optical phenomenon that results from wave vector matching between electromagnetic radiation undergoing total internal reflection from a thin SPR conductive metal surface and free electrons present at the thin SPR conductive metal surface. Sensors based upon SPR have been applied in a variety of applications in the fields of chemical, biochemical, biological or biomedical analysis. One such system, manufactured by Texas Instruments Incorporated™, is a low-cost, portable electronic sensor platform that determines the refractive index of a medium in contact with a thin conductive metal sensing surface as in described in U.S. Pat. No. 5,912,456, which is incorporated by reference herein. The sensitivity of this platform is confined to within approximately 200 nm of the SPR active surface which is ideal for a wide variety of chemical, biochemical and biomedical analysis applications. The design of the sensor facilitates large-scale production of low cost, disposable sensors. In a biosensor application, the absorption of biomolecules on the SPR active surface increases the refractive index which is monitored in real time by an electronic system supporting the optoelectric components of the sensor. The resulting interaction curve of refractive index change versus time, represents the binding profile of the biomolecule to the surface.
Specifically, an SPR sensor includes an electromagnetic radiation source, an optical aperture, a polarizer and detector array, all of which may be encapsulated in an optical medium, such as glass or transparent plastics, shaped to accommodate an optical path that supports SPR. Electromagnetic radiation from the source emerges from the aperture and is polarized to ensure that the transverse magnetic (TM) polarized component is retained while the transverse electric (TE) polarized component is eliminated. The TE polarized component is incapable of generating a resonance and is eliminated in order to reduce the fraction of non-interacting light reaching the detector array. The TM polarized light beam undergoes total internal reflection from the thin SPR conductive metal surface and reflects off of a mirror, which directs the reflected light onto the detector array. Resonance occurs when the wave vector of the incident TM polarized radiation matches the surface plasmon mode of the thin SPR conductive metal surface thus generating a charge density wave that adsorbs radiation at particular incident angles. The angular position of the resulting reflectance minimum is dependent upon the refractive index at the sensing surface and is detected by the detector array. The measurable range of the refractive index dependent resonance can be adjusted within certain theoretical limits by varying the range of incident angles employed. In addition, if an encapsulating optical medium is used, varying the refractive index of the encapsulating optical medium will adjust the measurable range of the refractive index dependent resonance.
Optical defects and temperature gradients cause time dependent interference in systems that use surface plasmon resonance. The time dependence of the interference severely reduces the effectiveness of the initial referencing and, hence, results in high noise levels. Conventionally, in an effort to reduce time dependent interference, a reference signal is obtained while the dielectric conditions adjacent to the thin SPR conductive metal surface foster an environment where no resonance occurs. To date this has been performed by exposing the sensing surface to a medium having a refractive index outside the SPR range for that particular sensor configuration. The signal obtained during resonance, when the sensing surface is exposed to the sample of interest, is divided by the non-resonance signal to give the corrected SPR signal. For example, exposing the sensing surface to air which has a refractive index of ‘1’, or to glycerol which has a refractive index of ‘1.42’, is appropriate for obtaining a non-resonance signal reading in a sensor having a refractive index measuring range from ‘1.333’ to ‘1.40’. This procedure is highly effective when the optical system is stable.
Problems, however, arise when fluctuations in temperature and mechanical stress exist that generate optical defects within the light path give rise to interference, due to light scattering effects, that are variable with time. Hence, the non-resonance signal must be updated continuously, denoting real-time referencing, in order to minimize these interferences. This is difficult to perform, since it is necessary to ensure that the sensing surface is continuously in contact with the sample of interest.
Thus, a need exists for a disposable, portable, miniature surface plasmon resonance sensor having the ability to take a reference at any time in the presence of any interaction, without changing the sensing surface condition. In addition, there exists a need for such a system that has the ability to reduce baseline noise, improving the quality of the system substantially. This sensor must maintain high sensitivity and detection of analytes and must be cost effective to manufacture.